1. Field of the Invention
The present invention relates to a novel method for preparing dihydric phenols, such as hydroquinone and catechol, useful as a raw material for several industrial chemicals, pharmaceuticals, agricultural chemicals and perfumes, etc.
2. Description of the Related Art
Methods for preparing dihydric phenols, such as a hydroquinone and catechol, which are produced by oxidizing phenols with hydrogen peroxide in the presence of many catalyst systems have been studied for a long time.
For example, the following references describe the oxidation of phenols in the presence of various catalyst: Japanese Patent Publication Kokai No. 50-130727 describes phenol oxidation in the presence of ketones, such as, 4-methyl-2-pentanone and methyl phenyl ketone, etc., and sulfates, such as an aluminum sulfate;
Japanese Patent Publication Kokoku No. 56-47891 describes phenol oxidation in the presence of strong acid mineral acids, such as a perchloric acid;
U.S. Pat. No. 3,914,323 describes a method known as the Fenton Method in which phenols are oxidized in the presence of iron or cobalt salts;
U.K. Patent No. 2,116,974 and EP Patent No. 314,582 describe a method in which phenols are oxidized in the presence of synthetic zeolites containing Ti atoms as a catalyst;
EP Patent No. 299,893 describes a method in which phenols are oxidized in the presence of a lamellar acid clay as a catalyst; and
EP Patent No. 132,783 describes a method in which phenols are oxidized in the presence of a strong acid type ion-exchange resin as a catalyst, etc.
All of the above methods produce a high yield of dihydric phenols versus hydrogen peroxide, but have a very serious problem i.e., for example, a hydroquinone and a catechol are co-produced when phenols are oxidized with hydrogen peroxide. That is, the para- to orth-isomer ratio (P/O) of dihydric phenols in the above cases using a ketoneperoxide catalysts system, mineral acid system and the Fenton reaction are 0.4-0.7; therefore, ortho-isomers, which are in little demand, are produced in large amounts compared with the amounts of para-isomers. For this reason, there is the drawback that the production amounts and the production costs of the para-isomers are very adversely influenced by the market for ortho-isomers, e.g., the market for catechols. EP Patent No. 314,582 discloses that an improvement of the para-selectivity can be accomplished by using a titanosilicate catalyst having the same crystalline structure as ZSM-5 zeolite; in this case, however, the P/O ratio is also around 1. In the case where lamellar clay is used (EP Patent No. 299,893), an example is set forth in which the P/O ratio was improved to 1.86; but the conversion ratio of hydrogen peroxide was small, and the method, therefore, is not practical. Further, there is a report that the P/O ratio can be improved to 12.5 when a strong acid type ion-exchange resin containing a cation of a transition metal, such as Ti or V etc., is used as a catalyst; the yield of dihydric phenols versus hydrogen peroxide was small, however, and the result, therefore, is unsatisfactory. A method of obtaining para-isomers, utilizing the shape selectivity of ZSM-5 zeolite (U.S. Pat. No. 4,578,521) has recently been disclosed. Although the present inventors tried to hydroxylate a phenol according to the method described in the above U.S. Patent, satisfactory yield and selectivity could not be obtained, and the method was not sufficiently reproducible.